1. Field of the Invention
The present invention relates to an antenna device and, in particular, to an antenna device suitable for multi-band operation. The present invention relates to an antenna for wireless data transmission, which may also include voice transmission.
2. Description of Related Art
For a wireless connection of mobile data processing devices, such as, for example, in wireless local area networks (WLAN), compact small antennas which often need to be dual-band- or multi-band-capable are required.
For this purpose, separate antennas may be used in practice for each frequency range. These separate antennas are, for example, connected to a diplexer in the form of a directional filter or to a multiplexer by means of which the signals to be transmitted are distributed to the respective individual antennas corresponding to the frequency ranges used. The disadvantage of using separate antennas for each frequency range is the size of the individual antennas, the area required for the antennas increasing with an increasing number of antennas required. Additionally, the required distributing circuit in the form of a diplexer or a multiplexer consumes a considerable amount of space.
Another known approach is to use antennas which have a very broad band or are multi-band-capable. In Kin-Lu Wong “Planar Antennas for Wireless Communications”, John Wiley and Sons, Inc., Hoboken, N.J., USA, 2003, pp. 26 to 53, several dual-/multi-band antennas in particular for being used in wireless local area networks are explained. Integrated IFAs (IFA=inverted F antenna) and PIFAs (PIFA=planar inverted F antenna) are, among other things, described there.
Dual-band PIFAs described in the above-mentioned document include, on a main surface of a substrate, different antenna patches realized by slots in an electrode formed on the surface, the antenna patches being fed via a common feeding point and connected to ground via a common short-circuited point. Antennas of this kind are also described in Zi Dong Liu et al., “Dual-Frequency Planar Inverted F Antenna”, IEEE Transactions on Antennas and Propagation, Vol. 45, No. 10, October 1997, pp. 1451 to 1458.
This document by Kin-Lu Wong (pages 226 ff.) also describes an integrated dual-band antenna in the form of a stacked IFA antenna. Two IFA antennas are “stacked” and galvanically excited via a microstrip line. This antenna may also be employed for wireless local area networks.
Additionally, dual-band PIFAs in which an antenna patch is galvanically fed by a feeding point, whereas a second antenna patch is fed by a capacitive coupling to the galvanically fed antenna patch, is described in the document mentioned. Antenna patches of this kind having capacitive coupling are also described in Yong-Xin Guo et al., “A Quarter-Wave U-Shaped Patch Antenna With Two Unequal Arms for Wideband and Dual Frequency Operation”, IEEE Transactions on Antennas and Propagation, Vol. 50, No. 8, August 2002, pp. 1082 to 1087.
Another way of implementing a dual-band antenna in which the antenna patch is lengthened or shortened in a frequency-selective way via an LC resonator or a chip inductor connected therebetween, is also known from the above-mentioned document by Kin-Lu Wong and also described in Gabriel K. H. Lui et. al., “Compact Dual-Frequency PIFA Designs Using LC Resonators”, IEEE Transactions on Antennas and Propagation, Vol. 49, No. 7, July 2001, pp. 1016 to 1019.
A non-planar broad-band antenna using a radiation coupling technique is described in Louis F. Fei et al., “Method Boosts Bandwidths of IFAs for 5-GHz WLAN NICs, Microwaves and RF”, September 2002, pp. 66 to 70. The bandwidth of the antenna is extended in a non-planar integrated IFA antenna by means of the radiation-coupled resonating of another IFA antenna.
It can be denoted in general that IFA antennas most often have a greater bandwidth compared to PIFA antennas, wherein most integrable dual-band concepts are of disadvantage due to a smaller bandwidth or due to an increased area demand.